Development of fuel cells capable of high-efficiency power generation even in a small unit and using hydrogen gas or a gas containing hydrogen as fuel is being pursued to develop the fuel cells as a power generation system for distributed energy supply sources. However, no general infrastructure for hydrogen gas or a gas containing hydrogen for fuel at power generation has been developed. There is, therefore, a need to utilize a raw material supplied from an existing infrastructure for supply of a fossil raw material such as city gas or propane gas, and to provide, in combination with a fuel cell, a hydrogen generation apparatus which generates a gas containing hydrogen by reforming reaction between the raw material and water.
The hydrogen generation apparatus typically includes ordinarily constituted by a reforming unit in which a reforming reaction between a raw material and water is caused, a converting unit in which water gas shift reaction between carbon monoxide and water vapor is caused, and a selective oxidation unit in which carbon monoxide is oxidized.
In the reaction units, catalysts suitable for the reactions are used. For example, a Ru catalyst or a Ni catalyst is used in the reforming unit, a Cu—Zn catalyst is used in the converting unit, and a Ru catalyst or the like is used in the selective oxidation unit. There is a suitable temperature in each reaction unit. In most cases, the reforming unit is used at about 650° C., the converting unit at about 200° C., and the selective oxidation unit at about 150° C.
When the operation of the hydrogen generation apparatus as described above is shut down, there is a need to purge the interior of the hydrogen generation apparatus with an inert gas in order to limit oxidation of and dew condensation on the catalysts provided in the reforming unit, the converting unit and the selective oxidation unit. However, it is not easy to keep an inert gas prepared at all times in the case where the apparatus is installed in a home. A method of using a raw material gas as an inert gas has therefore been proposed (see, for example, Patent Literature 1).
In an apparatus in Patent Literature 1, the supply of a raw material gas is stopped immediately after shutdown of the operation of a hydrogen generation apparatus; the interior of the apparatus is purged with water vapor; cooling is performed until the temperature of converting catalysts in the apparatus are reduced to predetermined temperatures; and water vapor is thereafter forced out with the raw material gas.
According to the shutdown method in Patent Literature 1, however, the reforming catalyst is exposed only to water vapor at a high temperature for a certain time period and there is, therefore, a possibility of the reforming catalyst being degraded.
A method has been disclosed in which after a stop of the supply of a raw material and water at the time of shutdown, natural cooling is performed by standing for a certain time period without supplying water vapor, and the interior of the apparatus is thereafter purged with a raw material gas (see, for example, Patent Literature 2).